Component Object Model (COM) is a binary-interface standard for software components introduced by Microsoft in 1993. It is used to enable inter-process communication object creation in a large range of programming languages. COM is the basis for several other Microsoft technologies and frameworks, including OLE, OLE Automation, Browser Helper Object, ActiveX, COM+, DCOM, the Windows shell, DirectX, UMDF and Windows Runtime.
The essence of COM is a language-neutral way of implementing objects that can be used in environments different from the one in which they were created, even across machine boundaries. For well-authored components, COM allows reuse of objects with no knowledge of their internal implementation, as it forces component implementers to provide well-defined interfaces that are separated from the implementation. The different allocation semantics of languages are accommodated by making objects responsible for their own creation and destruction through reference-counting. Type conversion casting between different interfaces of an object is achieved through the QueryInterface method. The preferred method of “inheritance” within COM is the creation of sub-objects to which method “calls” are delegated.
COM is an interface technology defined and implemented as standard only on Microsoft Windows and Apple’s Core Foundation 1.3 and later plug-in application programming interface (API). The latter only implements a subset of the whole COM interface. For some applications, COM has been replaced at least to some extent by the Microsoft .NET framework, and support for Web Services through the Windows Communication Foundation (WCF). However, COM objects can be used with all .NET languages through .NET COM Interop. Networked DCOM uses binary proprietary formats, while WCF encourages the use of XML-based SOAP messaging.
COM is very similar to other component software interface technologies, such as CORBA and Enterprise JavaBeans, although each has its own strengths and weaknesses. Unlike C++, COM provides a stable application binary interface (ABI) that does not change between compiler releases. This makes COM interfaces attractive for object-oriented C++ libraries that are to be used by clients compiled using different compiler versions.